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Scan Cycle

Structured Text execution in truST is cycle-driven.

Term Meaning
%I Input process image.
%Q Output process image.
%M Memory-marker process image.
VAR_CONFIG Configuration-time mapping from ST variables to addresses.
Process image Sampled I/O/memory state a cycle reads and writes.
flowchart LR A[Sample Inputs
%I / driver image] --> B[Materialize Mapped Variables
VAR_CONFIG and globals] B --> C[Execute Structured Text
programs and function blocks] C --> D[Write Outputs and Memory
%Q / %M images] D --> E[Publish Results
HMI, watch, control, traces] E --> A

Figure: One truST scan cycle. Inputs are sampled first, logic executes against that image, and observers see the committed post-cycle state.

One Cycle, In Order

  1. inputs are sampled into the process image
  2. mapped variables are materialized for the running program/task
  3. Structured Text logic executes
  4. updated outputs and memory images are written back
  5. watchers, HMI, and control APIs observe the resulting state

Common Bug Source

Many PLC bugs are really scan-order misunderstandings. The docs, runtime panel, and deterministic harness all try to make that ordering visible instead of hiding it.

%M / memory-marker behavior

The memory-marker tutorial demonstrates the intended mental model:

  1. cycle start: %M process image is read into bound variables
  2. program logic mutates those variables
  3. cycle end: updated values are written back to %M

That is why memory_marker_counter shows a value latched from cycle start while the written-back %M value already reflects the increment for the next cycle.

Tasking

[resource] defines the main cycle interval. Optional [[resource.tasks]] entries let you override interval/priority/program groups for more explicit scheduling.

Why Determinism Matters

truST keeps this model visible so you can reason about:

  • why a value changed one cycle later than you expected
  • whether an input was sampled before or after a state transition
  • how tests should assert outputs over time instead of only at one instant
  • how a reload should preserve or reset state

When a tutorial or harness example steps cycle-by-cycle, it is teaching this execution contract directly.

Where You See The Scan Cycle

  • the runtime executes it continuously
  • the deterministic harness lets you step it manually
  • tutorials use it to explain %M and I/O behavior
  • HMI, traces, and watch surfaces expose the resulting state after each cycle